There are other downstream consequences.
Consider in a simple model two cities, A and Z, which are next to each other.
In A, the city focuses on high-end housing. It's successful. Rich people move to A. Rents go up on older housing, to meet the market demand, including the temptation to sell the plot, raze the structure, and build new high-end housing.
As a result, poorer people move to the more affordable Z. This is exactly what's predicted in your #1.
Since the expensive housing is in A, that city gets more tax money, which goes to schools, libraries, parks, etc.
Z doesn't have that revenue source, so ends up with worse quality schools, libraries, parks, etc.
The citizens of city A all have cars, and don't like all the non-residents coming through and using "their" libraries, parks, etc. so convert most streets to cul-de-sac versions and don't provide mass transit.
The citizens of Z, many with service worker jobs in A, must therefore buy cars in order to keep their jobs. Which lowers their overall income, and makes it harder to save up money.
Z responds by building more low-cost housing, but this is further away from A (where the jobs are), and doesn't address the unequal tax base.
> But isn't it obvious?
Well, yes. That is my point. I pointed out that if you assume you have a solution, then of course there's a solution. Remember, you argued "You could have the most evil landlords, corrupt politicians, and aloof newcomers, and an economic solution would still fix the problem." What economic solution is there if you have normal people? Not saints, and not demons.